Alternating current power control



Feb. 27, 1968 K. J. KNUDQSEN $371,268

ALTERNA'IING CURRENT POWER CONTROL Filed Feb. 8, 1965 INVENTOR.

K ud Knucbaen United States Patent 3,371,268 ALTERNATING CURRENT PGWER(JONTRGL Kuud .l'. Knudsen, Daytona Beach, Fla, assignor to LewisEngineering Company, New Haven, Conn, 21 corporation of ConnecticutFiled Feb. 8, 1965, Ser. No. 431,040 9 Claims. (Cl. 323-21) ABSTRAfJT OFTHE DHSCLOSURE An alternating current power control comprising a bridgecircuit which responds to the power output of an electrical load. Thebridge circuit in turn controls a photosensitive device having avariable output voltage which, through a gated SCR and suitabletransformer, controls the power to the load. A lamp of thephoto-sensitive device is energized from the source that provides theelectrical power, and the light output of the lamp is kept steady by avoltage control which utilizes a Zener diode.

This invention relates to electric control devices, and moreparticularly to A.C. electric power control equipment.

The invention concerns improvements in the electric power control devicewhich is disclosed and claimed in my Patent No. 3,116,396 dated Dec. 31,1963 and entitled Electric Temperature Control.

An object of the present invention is to provide a novel and improvedAC. power control device which is especially sensitive and accurate inmaintaining, by means of a photo-sensitive control element andincandescent light source associated therewith, a given condition as forexample the temperature of an electricallyheated bath orliquid-containing vessel.

Another object of the invention is to provide an improved power controlas above set forth, wherein a simplified and effective anticipator meansis provided to narrow or'bring closer together the limits of thetemperature swing as effected by the control.

A further object of the invention is to provide an improved controlhandling appreciable power and involving a light-sensitive element and alight source associated therewith in conjunction with a sensingenergized bridge, wherein energization of the light source andthe-bridge is obtained from the power line supplying a heavy load andwherein fluctuations of the light source due to variations of the loadand energizing power are largely eliminated or held to an absoluteminimum, thereby increasing the accuracy of the control.

An additional object of the invention is to provide a novel andadvantageous contactless AC. power control for an electrical load in therange including several kilowatts, wherein a minimum number ofcomponents of simple construction is required, and wherein relativelylarge currents are switched by small values of voltage and power.

Yet another object of the invention is to provide an improved A.C. powercontrol in accordance with the foregoing, wherein the anticipator meansis directly responsive to the heavy load current without theintermediary of intervening circuits which are primarily relied on forother functions.

Features of the invention involve the provision of an improved A.C.electric power control which is relatively small and compact as regardsits physical size, especially simple and requiring relatively few parts,which is sturdy and reliable in its operation, and wherein the maximumnumber of elements and assemblages are rugged and resistant tovibration, rough handling, and the like.

' Other features and advantages will hereinafter appear.

The sin le figure of the drawing is a schematic circuit diagram of theimproved AC. power control device as provided by the invention.

Referring to the drawing, heavy current power supply terminals areindicated at L1 and L2, from which the system is energized. Additionalterminals 11 and 12 are shown, for connection respectively to theterminals L1, L2 to receive energy therefrom simultaneously. A c0ntrolbridge designated by the numeral 14 is shown within a dotted-outlinebox.

The terminals ll, 12 provide current for a DC. power supply designatedgenerally by the numeral 16 and which is also outlined by a broken-linebox. The supply 16 provides energy for the bridge 14 as well as for alowcurrent control means by which the heavy current flowing through theterminals L1 and L2 is controlled, to effect a control of a high powerload.

In the illustrated embodiment of the invention, the load indicated at 18comprises an electric heater or coil which may be rated in kilowatts.The heater i8 is associated with a bath or container of liquid 29, andthere is disposed in heat-receiving relation with the heater 18 or thebath Zti a sensing element 21 of the bridge 14. Where a close control ofthe temperature of the bath 20 is required, the sensing element 21 maybe immersed in the bath rather than being associated directly with theheater 1%.

The power circuit connected to the terminals L1, L2 includes a currentcontrol device for the load 18, which device is responsive to variationsof a small control voltage. Such current control device is illustratedherein as an auto transformer 22 having a coil 23 connected by leads 24,26 respectively to the terminal L2 and the heater 18. The heater in turnis connected by a lead 28 to the terminal L1. The auto transformer 22has another winding portion 30 connected by a wire 32 to the base 34 ofa gate-type silicon-controlled rectifier 36 having a gate 38 andcollector it). The collector 4%) is connected by a wire 42 to the powerlead 24, which is grounded at 44.

Q When the SCR is not conducting, the auto transformer 22 represents ahigh resistance or impedance in the power circuit whereby no substantialload current flows therethrough, thereby effecting a de-energization ofthe heater 18. If the SCR 36 should be rendered conducting, as by theapplication of a positive or more positive (with respect to ground)voltage to the gate 38, this will result in the circuit having the autotransformer and which is included between the power lines 24, 26 showinga very low impedance, whereby a heavy full wave energizing currentpasses through the load or heater 18. The SCR 36 will conduct in onlyone direction, but with the circuit provided full wave current will besupplied to the load, as will be later brought out in detail.

Thus, the auto transformer 22 and the SCR 36 in the arrangement shownconstitutes in effect a switching or relay device which is controlled bysmall positive potentials applied to the gate 38. It will be understoodthat an alternating current supply is connected to the terminals L1, L2,and any one of alternate half waves of such alternating current has theeffect of switching off the SCR 336 for such waves. Whenever thepositive potential is removed from the gate 38, the said switching offremains in effect for all alternations. With an AC. supply and positivepolarization of the gate 38, the control will be conducting for bothhalf cycles of the alternating current, providing energization of theheater 18. As long as a positive potential is being applied to the gate38, conduction will occur through the heater,

The action in the control 22, 36 is as follows: If the gate 38 is notpolarized to render the SCR 36 conducting,

3 the eifect of the SCR between the wires 32 and 42 will be that of anopen circuit. The transformer 22 will have its flux reversing with thecurrent alternations, presenting a high impendance in the circuit withlittle current and substantially no heater energization.

When the gate 38 is polarized to render the SCR conducting, conductionwill occur during alternate half cycles, and the remaining half cycleswill switch off the SCR despite the gate polarization. Such conductionduring the first-stated half cycles will pass half cycles of the loadcurrent, energizing the heater 18; also, the heavy load'current in thecoil 30 of the transformer during such half cycles will cause saturationof the core in one direction despite an opposing tendency of smallercurrent in the coil 23. For the remaining half cycles, during which theSCR 36 is not conducting, heavy load current will flow through thetransformer coil 23, again energizing: the heater 18- (whereby thelatter is energized with full wave current). The reason for this heavyload current during the remaining half cycles is that the transformercore as previously saturated, is maintained in saturated condition bythe reversal current (of the cycle) now fiowin g in the coil 23'. Thiscore saturation hasthe effect of enabling the coil 23 to present a lowimpedance during said remaining half cycles. The core saturation fluxdoes not change direction at all during operation of the SCR because thealternate flows of current occur in different coils of the transformer.For example, the dominating flow in coil 30 could be downward foralternate halft cycles, and also downward in coil 23 for the remaininghalf cycles.

The control voltage (which is made to be automatically responsive tochanges in the temperature of the bath 20) is obtained, in accordancewith the invention, from a sensitive and accurate light-responsive meanscomprising a conductive-type photoelectric cell 46 which has thecharacteristic of changing from a relatively high resistance to a verylow resistance when excited by light. The conductive cell 46 has oneterminal connected to a-wire 48 which is connected to the gate 38 of theSCR 36. The cell is also connected with drop resistors 50 and 52energized from the DC. power supply 16.

The resistors 50, 52 are connected respectively to rectifiers 54, 56which are in turn connected by wires 58, 60 to one end and a tap 61 ofthe secondary coil 62 of a power transformer 64 having a primary 66connected to the supply terminals 11, 12. The terminal 12 is alsoconnected to a ground 68 or common negative lead 68, and such ground isconnected by a wire 70 to the remaining end terminal ofthe secondarycoil 62.

A filter or smoothing capacitor 70 has its positive terminal connectedto the juncture of the resistor 50 and the rectifier 54, and has itsnegative terminal connected by a wire 72 to a ground 74 or commonnegative path.

By such organization, a DC. voltage exists between the wire 71 (which ispositive) and a Wire 57 which connects the resistor 52 with therectifier 56. When the photocell 46 is not illuminated it has a veryhigh resistance, resulting in the wire 48- having a small negativepotential which depends for one thing on the voltage of the powersupply. Such small negative potentialmay, for example be volts, and thecorresponding positive potential on the wire 71 may be volts,representing a potential difference of: volts as obtained from the powersupply 16. This assumes negligible voltage drop through the resistor 52.

When strong light impinges on the photocell 46 it will render the latterconducting. Assuming a bright illumination, the resulting conductiveconditions of the cell 46 may place a positive potential of 3 volts onthe, wire 48, which then is impressed on the gate 38 of the SCR 36. Suchpositive potential will result in the current control device 22presenting a low impedance in the power circuit, whereby the load 18 issubstantially energized, causing heating of the bath 20.

On the other hand, the 5 volts resulting from the darkened condition ofthe photocell 46 will enable the 4- SCR 36 to cut off, thereby shuttingoff the power to the load 18.

The contactless control as provided by the invention and set forth aboveis thus seen to be advantageous in handling loads in the range includingseveral kilowatts, since it has relatively few components of simpleconstruction, rugged character and greatreliability, and since heavycurrents may be switched readily, utilizing small values of voltage andpower.

In accordance, with the invention, an accurately controlled beam oflight of constant intensity is utilized to excite the photocell 46, suchbeam being substantially independent of nominal fluctuations in voltageof the energy source which supplies the terminals L1, L2, in spite ofthe fact that the DC. power supply 16 receives its energy from the samesource.

The electric light source comprises anincandescent lamp 76 which,together with the bridge 14, is energized from the DC power supply 16.The lamp 76 has one terminal connected by a wire 78 to a ground 80 andits other terminal connected by a wire 82 to a droppingresistor 84-which is in turn connected to the positive supply line 86 from'thesupply 16. The line 86leads from a rectifier 88 and a filter orsmoothing capacitor 90, the latter being connected to a ground 92. Therectifier 88 is connected by a wire 9410a capacitor 96 of high value,which is in turn connected by a wire 98 to the transformer secondary 62at the same terminal which is joined to the wire 58. A- rectifier 100 isconnected with a ground 102 and by a wire 104 to the wire 94, therebycompleting the circuit of'the power supply 16.

The bridge 14 comprises,.besides the sensing element 21', paralleledresistors 106, 108 having end connections to resistors 107 and 109,these latter two resistors constituting two bridge legs. The resistor109 connects also with the wire 86, and the resistor 107 is grounded at126. The resistor 108 has a slider 110 connected by a wire 112 to ananticipator resistor 114. The resistor 114 has a slider 116 connectedwith a. light-control means in the form of a; galvanometer or DArsonvalmovement 118 bridged by a capacitor 120 and connected by a wire 122 tothe junc ture' ofthe sensing element 21 and a resistor 124, these twoconstituting other legs of the bridge. The resistor 124 is joined to thewire 86. The sensing element 21 is connected'to' a ground 130.

The galvanometer 118 on its movable system may have a mirror 132 whichreflects light from the lamp 76 and directs the light against thevphotocell'46, or an opaque vane may beused as in my patent aboveidentified. With the above arrangement an automatic control is had ofthe energization of the load 18, inasmuch as the sensing element 21 ofthe bridge is responsive to changes in temperature of the bath 20 heatedby the load 18. Such response of the sensing element will causedeflection of the galvanometer1-18 and will either direct light againstthe photocell46- or else direct the light away from such cell, therebyto effect an automatic control of the energizing current for the load 18and in the manner of well known controls of this type (as explained alsoin my above identified patent).

In accordance withthe present invention, in conjunction with suchautomatic control I provide a novel and improved, simplified yeteffective and reliable anticipator device which is designated generallyby the numeral 134 and which includes the anticipator resistor 114mentioned above. The anticipator device comprises also a currenttransformer 136 having a heavy current primary 138 in the line 26 andhaving a light current secondary 140, the latter being connected to theresistor 114- and also by a wire 142 to a rectifier 144 which is in turnconnected to the remaining terminal of the resistor 114. Also, anadditional rectifier 146 is connected across the transformer secondaryto prevent rectifier 144 from breaking down on the negativealternations. The conductive directions of the rectifiers are asindicated.

With such arrangement, for alternate half cycles of the alternatingcurrent wave, the rectifier 146 acts as a shortcircuiting device,preventing any appreciable voltage from existing across the resistor 114and rectifier 144. Thus, for such half cycles of the wave, no effect isproduced on the galvanometer 118. The short-circuiting of the winding140 by the rectifier 146 is not such as to cause excessive heat in ordamage to the winding, since low voltages are involved and since theimpedance and resistance of the heater winding 18 limits theshort-circuiting current to a substantially inappreciable value asregards damaging heat.

For the remaining half cycles, current will flow from the secondary 140through the rectifier 144 and the resistor 114, and in consequence thevoltage drop of the resistor, as measured to the slider 116, will beincluded in the circuit of the galvanometer 118, causing a deflection ofthe latter in a direction which tends sooner to initiate the requiredcontrol of the power circuit 24, 2d. That is, if the load current is cutoff and the bath 2.0 is cooling, the response of the sensing element 21will change the bridge balance in such a manner that the galvanometerdeflection results in light striking the photocell 46. This will place apositive voltage on the wire &8 and render the SCR 336 conducting, andthe resultant heavy load current in the line 24, 26 which now energizesthe load 18 will also cause a voltage at the resistor 114- which is in adirection tending to increase the light being directed against thephotocell 46. Therefore a larger swing of the galvanometer 118 is hadthan would otherwise occur if no anticipator device were provided.

Conversely, when the sensing element in responding to the increased heatnow changes the bridge balance to oppositely deflect the galvanometer soas to shut oil the light from the photocell 46, the resultant cessationof the load current in the power line 24, 26 will shut oil? the voltageacross the anticipator resistor 114, thereby tending to increase thesaid opposite deflection of the galvanometer 118 and directing the lightbeam further away from the photocell 46.

The anticipator 134 thus provides for a greater response of thegalvanometer, and results in a more effective and accurate control. Byvirtue of the load current .being directly utilized through thetransformer 136 to provide these desired exaggerated actions of thegalvanometer, the control is made especially effective as compared withanticipator devices wherein intermediary equipment is involved inproviding the anticipator voltage.

In the present instance, the anticipator voltage is supplied from adevice which functions purely as an anticipator and is required to carryout no other function.

Also, in accordance with the invention, the illumination of the lamp 76is rendered substantially independent of fluctuations of the powersource and voltage supplying the power terminals L1 and L2. This iseffected by the provision of the dropping resistor 84- and also by theprovision of a voltage-responsive and limiting means comprising a Zenerdiode 150 which is connected across the terminals of the lamp 26 asshown. The Zener diode 150 has one terminal grounded at 152 to effectsuch shunt circuit, and is so arranged that in conjunction with thenominal voltage (for instance volts) from the supply 16 and the dropthrough the resistor 84 it maintains a constant applied voltage to thelamp 76. For example, the lamp 76 may have a rating of 5 volts and theZener diode may have a rating of 4.7 volts and a capacity of 400milliampers. The resistor 34 may be 90 ohms, and the line 85 may have anominal voltage of +15 with respect to ground. The energization of thelamp 76 will thus be held closely to the value of 4.7 volts regardlessof nominal fluctuations in the 15 volt output from the power supply 16.Wide variations in the output voltage of 15 from the supply 16 may occurwithout changing the 4.7 volts applied to the lamp 76. Thus, theintensity of the light is maintained at a constant value regardless ofvoltage changes of the system, and this results in a more effective andaccurate control of the load current being had. Before the brilliance ofthe lamp 76 drops or decreases, the

voltage of the line 86 would have to drop to a value lower than therating of the Zener diode 150, that is, below approximately 4.7 volts.This, of course, .is a condition which is very unlikely to occur duringthe normal operation of the control and power unit.

Besides the values given above, the various components shown in thedrawing are further characterized as follows: The load 18 may be ratedat 2 kilowatts, for 120 volts AC. Power supplied to L1, L2 may be 115volts at 18 amperes RMS. The primary coil 138 of the anticipatortransformer may comprise a single turn of No. 10 copper wire, and thecoil 140 may be constituted of the primary of a Stancor transformer No.P6134. The anticipator resistor 114 may be 6 ohms. The SCR 36 may berated at 18 ampers, type 2N688. Capacitor 120 may be 250 mfd. Resistors50 and 52 may be respectively300 ohms and 2,000 ohms. Capacitors 96 andmay be respectively 1,000 mfd. and 500 mfd. Resistors 124 and 199 may beeach 1,800 ohms. Resistors 106 and 108 may be respectively 360 ohms and200 ohms. Sensing element 21 may be aproximately ohms at 25 C., and 247ohms at 300C. The rectifiers may be type 1N4001. Diode 100 may be aZener diode for voltage regulation, rated at 15 volts. Resistor 107 maybe 110 ohms.

It will now be seen from the foregoing that I have provided a novel andimproved, simplified yet highly effective and acurate automaticalternating current power control employing a light-responsive element,wherein accurate contactless control is effected with few and simplecomponents, wherein the light intensity is maintained at a constantvalue regardless of voltage fluctuations, and wherein the controlgalvanometer, by means of a simplitied and effective anticipator device,is made to have an increased movement in response to flowing andshut-off of the power current.

Variations and modifications may be made within the scope of the claims,and portions of the improvement may be used without others.

" I claim:

1. In an automatic electrical in combination:

(a) a power circuit having an electrical load and a current controldevice for said load, responsive to variations of a small controlvoltage,

(b) light-responsive means providing a small variable control voltagefor said control device,

(0) an electric light source and means for energizing the same from saidpower circuit,

(d) light control means responsive to changes in the load currentthrough said power circuit, for directing more or less light from saidsource to said lightresponsive means, thereby to establish an automaticcontrol of energization of said electrical load, and

(e) a voltage-responsive, voltage limiting device connected to thelight-source energizing means, limiting the maximum voltage applied tothe light source to a value close to its rated voltage, said energizingmeans being arranged to deliver a voltage higher than the rated voltageof the light source, thereby to minimize variations of said source withchanges in the loading of the power circuit.

2. A device as in claim 1, wherein:

(a) the means for energizing the light source provides DC. power forsaid source, and

(b) the voltage-limiting device comprises a Zener diode connected acrossthe light source.

3. A device as in claim 2, wherein:

(a) a drop resistor is connected in series with the parallel circuit ofZener diode and light source.

4. A device as in claim 1, wherein:

(a) the light control means includes an energized electric bridgereceiving its power from the said means for energizing the light source,said bridge having a supply conductor connected with the said lightsource.

power consuming device,

5. In a control device, in combination:

(a) an A0. power circuit having' an electrical load and a currentcontrol device for said load;

(1)) a BIG. bridge including a sensing element acted on by said load,

('c).' said bridge having a circuitwhich includes anelectrically=operated control mechanism energized by direct current ofthe bridge;

(d) means responsive to load current changes in the power circuit andcomprising an anticipator device connected with said power circuit,providing a small DC. voltage which fluctuates with load current in saidcircuit, and

(e)' means connecting said anticipator device to thecontroliinechan'ism'ofthe bridge ,to'operate the latter in the samesense as the operation thereof in response to action on the sensingelement by the said load.

6'; A'device as in claim 5, wherein:

(a)"sai'd anticipator device comprises a couplingmeans energized byAC.from the power circuit and a rectifier circuit connected to saidcoupling means andco'nt'rol mechanism and converting said A.C. to-D.-C.

7 for use by the latter.

7; A device as inclaim 6, wherein:

(a) said coupling means comprises a transformer having a primaryconnected in series with the power circuit and having a secondarybridged bythe rectifier circuit,

(b) said rectifier circuit comprising parallel branches. constituted ofa rectifier whichis connected acrossa series-joined rectifier andvoltage divider resistor,

(c') said rectifiers having one set of unlike terminals connectedtogether.

8. An A.C. electric power control device comprising, 35

in combination:

(a) asaturable core and two windingson said coreconnected in series, (b)a gate control type SCR device having: collector and base connected to'the free ends of the windings, (c) means providing a DC. controlpotential: applied".

to the gate of the SCR device to render the same either conducting ornon-conducting, and

(d) an energized load circuit including an electrical load, said circuithaving leads connected respectively to the series connection of saidwindings and to the collector of the SCR device,

(e) saidcore remaining saturated with flux in one direction for a givenhalf wave of power when the SCR is provided with a control potential toeiiect its conduction and is conducting power pulses in one directionwhereby the load circuit has included in it a low impedance to increasethe current therein,

(f) said core remaining saturated with flux in said one direction forthe succeeding half wave of power when the SCR is non-conducting whilestill provided with a control potential to efi'ect its conduction, and

(g) said core having an alternating flux when the SCR is provided with acontrol potential to establish its non-conduction and is non-conductingwhereby the load circuit has included in it a high impedance to greatlyreduce the current therein and through the load.

9; A power control device as in claim 8, wherein:

(a) the means providing a control potential applied to the gate of theSCR comprises a photo-sensitive cell and a light source adapted toactivate said cell,

(b) said photo-sensitive cell being connected to the gate of the SCR andapplying a more positive potential to said gate in response to the cellbeing activated.

References Cited UNITED STATES PATENTS Re.261,1l9 6/1960 Slater 32322JOHN: F. COUCH, Primary Examiner.

WARREN E. RAY, Examiner.

8/1956 Knudsen 219-20

